Shallow water riser system

ABSTRACT

A conduit structure ( 44 ) connects subsea oil wells to a floating structure ( 12 ) such as a vessel, in shallow water, the conduit structure providing a low cost and reliable fluid connection during drift of the vessel. The conduit structure includes a seafloor riser support ( 50 ) with a lower end ( 52 ) fixed to the seafloor and an upper end ( 54 ) lying a plurality of meters above the seafloor. A flexible pipe or hose ( 46 ) extends in a double catenary curve from the top of the seafloor riser support, at a downward incline away from the seafloor riser support and then at an upward incline to the floating structure. A rigid pipe ( 70 ) can extend along a plurality of meters of the height of the riser support to minimize the required length of flexible hose and facilitate installation.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] Applicant claims priority from U.S. provisional application No.60/352,751 filed Jan. 30, 2002.

BACKGROUND OF THE INVENTION

[0002] One type of system for producing hydrocarbons from underseareservoirs of limited capacity, includes a floating structure such as avessel anchored by catenary chains to the seafloor, or spread moored, orotherwise moored in a manner that allows limited vessel drift.Hydrocarbons from a seafloor well tapped into the reservoir, flowthrough conduits of a conduit structure, that extend up to the vessel tofill tanks in the vessel. Fluids such as injected gas may be pumpeddownward through a conduit back into the reservoir. Additionalconnections such as electrical and hydraulic connections may extend fromthe vessel to apparatus at the seafloor. The conduit structures mustcontinue fluid connections between the vessel and seafloor well(s)despite drifting of the vessel within a limited drift zone. The conduitsshould not hit the mooring chains or the seafloor, since this can causewear of a conduit.

[0003] One prior art conduit structure includes a first flexible hosethat extends almost vertically up from the seafloor to an underwaterbuoy, and a second flexible hose that extends in a double catenary curvefrom the buoy to the vessel. In moderate to deep water (e.g. about 100meters or more) the buoy lies high above the seafloor and the doublecatenary second hose provides a connection during vessel drift. However,a considerable length of hose is required, and flexible hose isexpensive and not as reliable as a fixed pipe. In shallow water, anyunderwater buoy must lie close to the seafloor, resulting in appreciablecost for the buoy, for a heavy seafloor weight to moor the buoy, and forhose connections of a short first hose. In addition, a buoy at shallowdepths moves sideward in heavy waves, in directions that may be counterto vessel movement, and the moveable parts limit the reliability of abuoy—based conduit system in shallow water. A fluid transfer system fortransferring fluids between a seafloor structure and a floatingstructure in shallow water, which was of minimal cost while providingreliable connections during vessel drift, without a conduit beatingagainst an anchor chain or the seafloor, would be of value.

SUMMARY OF THE INVENTION

[0004] In accordance with one embodiment of the present invention, anoffshore fluid transfer system is provided, of the type wherein aconduit structure that includes a flexible pipe or hose, connects aseafloor structure such as an undersea reservoir to a floating structuresuch as a vessel, which minimizes the cost of the conduit structure inshallow waters. The conduit structure includes a rigid seafloor risersupport with a lower end mounted on the seafloor and an upper end, and aflexible hose that extends from an upper portion of the seafloor risersupport in a double catenary curve to the floating structure. A rigidpipe preferably extends a plurality of meters along the riser support.The seafloor riser support minimizes the cost of the lower portion ofthe conduit structure while increasing its reliability. The top of theriser support can be wide and have a convex upper surface, to allow thehose to be lifted off and placed back on the upper surface.

[0005] The novel features of the invention are set forth withparticularity in the appended claims. The invention will be bestunderstood from the following description when read in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a side elevation view of a shallow water riser system ofone embodiment of the present invention.

[0007]FIG. 2 is a side elevation view of a seafloor riser support of thesystem of FIG. 1.

[0008]FIG. 3 is a rear elevation view taken along arrow 90 of FIG. 2.

[0009]FIG. 4 is a partial side elevation view of a fluid transfer systemof another embodiment of the invention, wherein a seafloor riser supporthas a convex upper surface and the flexible hose carries weights.

[0010]FIG. 5 is a sectional view of a portion of the conduit of FIG. 4.

[0011]FIG. 6 is a side elevation view of the seafloor riser support ofthe system of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012]FIG. 1 illustrates an offshore fluid transfer system 10 thattransfers fluids such as hydrocarbons, between a compliantly anchoredfloating structure 12 such as a vessel, and a seafloor structure 14. Theseafloor structure 14 is connected to seafloor wells leading to anundersea reservoir 16, and is anchored to the seafloor 18. The vessel 12includes a turret 22, and includes a hull 20 that is pivotable about alargely vertical axis 24 about the turret. The turret can lie within thehull or outward of the hull. The vessel, which floats at the sea surface26, is moored through a mooring system 30 that includes a plurality oflines such as cables or chains 32 that extend in catenary curves to theseafloor 18 or that are always under tension. Other mooring systems canbe used for a floating structure, such as a spread moored system thatprevents weathervaneing (rotation) of the vessel so a turret is notrequired.

[0013] Fluid such as oil and gas from the undersea reservoir flowsthrough conduits 42 of a conduit structure 44. The conduits includeflexible risers 46 in the form of flexible pipes or hoses that may bereferred to as flexible conduit members. It is noted that in someapplications, fluids can flow between a pipeline on the seafloor and thevessel.

[0014]FIG. 1 shows the vessel 12 in a quiescent position, which itassumes in a calm sea. Under the forces of winds, waves and currents,the vessel can drift from its quiescent position. The drift zone withinwhich the vessel can drift, is calculated for weather conditionsexisting at the particular location. During such vessel drift, the upperends of the flexible risers, or hoses, follow the vessel while otherportions of the flexible risers bend and translate.

[0015] The vessel lies in a shallow sea of height A which is generallyno more than about 200 meters and usually no more than 100 meters. Theconduit structure 44 is designed to allow the flexible risers 46 tofollow the drifting vessel, in a conduit structure of simpleconstruction, low maintenance, and low cost.

[0016] The conduit system includes a substantially rigid seafloor risersupport 50 whose lower end 52 is anchored to the seafloor and usually isrigidly fixed to the seafloor as by piles 56. The riser support 50 is arigid frame that has an upper end 54 that lies a plurality of metersabove the seafloor, preferably at least five meters and more preferablyat least ten meters above the seafloor. The height B is at least 10% ofthe seafloor depth A, and preferably at least 20% of the seafloor depth,and more preferably at least 30% of the seafloor depth. The flexiblerisers 46 extend from the upper end 54 of the seafloor structure indouble catenary curves to the vessel 12. Applicant uses the term “doublecatenary curves” to indicate that one portion 60 of the flexible risersextend at a downward incline from the upper end 54 of the seafloor risersupport to a lowermost point 62 along the risers (in the quiescent orstatic position of the vessel, which is illustrated), while anotherportion 64 of the risers extend at an upward incline from the point 62to the vessel. Such double catenary curve is known to provide highflexibility.

[0017]FIG. 2 is a side elevation view of the riser support 50, which isrigid, whose lower end 52 is connected to the seafloor preferably in afixed connection and whose upper end 54 lies a plurality of meters abovethe seafloor. The conduit 42 includes a rigid pipe 70 with a lower end72 adjacent to the seafloor (preferably within about one meter of theseafloor) and which may include additional pipe lengths 74 that extendalong the seafloor away from the structure. The rigid pipe 70 has a farend 76 which is close to the top of the pipe and which lies just beyonda curved rigid pipe section 80 that is preferably curved between 45° and135° and that is illustrated as curved about a quarter of a circle(90°). This results in the pipe far end 76 extending at a downwardincline away from the curved pipe section. The flexible riser 46 has aninner end 82 that is fixed to the far end 76 of the rigid pipe. A bendstiffener 84 that allows bending at only a large radius of curvature,may lie around the inner portion of the flexible riser 46 if required tocontrol motion at this connection point. As mentioned above, the riserextends in a double catenary curve from its inner end at 82 to thevessel.

[0018]FIG. 3, which is taken along arrow 90 in FIG. 2 which extends in alongitudinal direction M, shows that the seafloor riser support 50includes a plurality of rigid pipes 70 labeled 70A-70F, with six rigidpipes being shown. The six rigid pipes are spaced apart in a lateraldirection L. Each rigid pipe has a far end near the upper portion or end54 of the structure, which is connected to a flexible riser, in themanner shown in FIG. 2, with all risers extending to the vessel.

[0019] In the particular system of FIG. 1, the sea has a depth A ofthirty-six meters and the riser support 50 has a height B of fourteenmeters above the seafloor 20, which is more than 25% (actually 39%) ofthe seafloor height. The upper end 54 of the structure is low enough toprevent it from being hit by the vessel even in a rough sea and in thefully loaded position of the vessel, or any other vessel that is likelyto come into the vicinity of the vessel to which the conduit isconnected. A tall seafloor riser support 50 provides reliable supportfor the lower conduit portion because the support moves very little ifat all. The seafloor riser support is more reliable and of lower costthan a prior float based system in shallow water. Where the rigid pipe42 extends along a plurality of meters of the seafloor riser supportheight, it replaces some of the required length of more expensiveflexible risers 46 to further reduce costs. The higher the upper end 54of the seafloor riser support, the greater the allowable length of theflexible riser 46 of FIG. 1 and therefore the greater the allowablevessel drift zone.

[0020]FIG. 1 shows an umbilical riser arrangement 92 for electricalsignals, hydraulic fluid, etc. The arrangement includes a rigid post 94with a lower end fixed to the seafloor, and rigid pipes 95 extendingvertically along the post. Right angle elbows 96 at the top of the postconnect to the umbilical risers 98. The flexible umbilical riserstypically have a much smaller diameter than the diameter (e.g. 0.3meters) of the flexible risers 46.

[0021] The rigid post 94 is a variation of the seafloor riser support50, and is especially useful for instances where a single flexibleconduit is required. The seafloor riser support 50 also may be used forumbilical risers and the rigid post 94 that forms a simple seafloorriser support may be used for one or more risers.

[0022] It is noted that in the prior art, flexible piping hoses andumbilicals were used that extended from the seafloor up to an underwaterbuoy, and flexible hoses then extended from the flexible buoy in doublecatenary curves to a vessel. This is useful for deep seas. However, fora shallow sea, the undersea buoy cannot lie high above the seafloor, andthe considerable expense for buoy connections of a short length offlexible hose to such buoy and to the seafloor would increase the costand decrease reliability.

[0023] Rigid pipe such as 70 in FIG. 2 can be resiliently bent to only avery large radius of curvature such as five hundred times the outsidediameter of the pipe for steel pipe, to assure that the pipe is bentonly within in its elastic limits. Flexible pipes and hoses canelastically bend to a much smaller radius of curvature, depending uponthe construction of the particular hose, but almost always can bend to aradius of curvature less than fifty times the hose outside diameter. Thewalls of the flexible pipe or hose comprise a costly structure to permitrepeated resilient bending. The life of a flexible pipe or hose that isrepeatedly bent, is short, and it may have to be replaced every fewyears.

[0024]FIG. 4 illustrates another fluid transfer system 100 wherein aturret 102 lies outboard of the hull 104 of a vessel 106. The systemincludes a seafloor riser support 110 that is rigid, that has a lowerend mounted on the seafloor, and that holds rigid pipes 112 that extendupward from the seafloor. Connectors 114 connect ends of flexible risers(flexible pipes or hoses) 120 to the rigid pipes. The flexible risersextend in curves around the arched top 124 of the seafloor risersupport, and then extend in double catenary curves from point 126 to theturret 102 of the vessel. Applicant mounts weight modules 122 to theflexible riser 120 at locations spaced along the length of the flexibleriser. The weight modules, which may be formed of steel, undergo lessacceleration and less motion during severe storms. The weight modulesmay be used with any flexible riser portion.

[0025] Waves apply large forces to the vessel and to the risers instorms. The fact that the seafloor riser support and the flexible risersends at 124 do not move with the waves, avoids a situation where thevessel and lower end of the riser move in opposite directions during astorm.

[0026] The sea depth D in FIG. 4 is forty-eight meters and the seafloorriser support 110 extends up from the seafloor by a distance E of twentymeters, which is sufficient to be sure that the fully loaded vessel 106and other vessels that come to the vicinity of the reservoir do notstrike the structure 110. The seafloor riser support height E is over30% (actually 42%) of the sea depth D.

[0027]FIG. 6 shows that the flexible riser 120 has a portion 130 thatextends over the arched top 124 of the seafloor riser support 110. Alength K of the upper surface 126 of the riser faces primarily upwardlyand has a length K of over one meter and preferably a plurality ofmeters. The flexible riser can lift off and fall back onto this andadjacent portions of the riser top. When the vessel drifts far away fromthe riser support (but within the drift zone), the riser portion 130 canlift off the arch and later fall back onto the arch. The risers may bebiased back to their illustrated quiescent position. The radius ofcurvature J of the arched top is preferably at least five times thediameter of the flexible pipe or hose (about 0.3 meters) and preferablymore than one meter and more preferably a plurality of meters. Thelongitudinal M length G of the arched top is a plurality of meters, thearched top in FIG. 6 having a length G of 9.5 meters and a radius ofcurvature J of 4.7 meters.

[0028] It is noted that FIG. 4 shows a system where the bottom of thedouble catenary curve 132 lies considerably above the seafloor in thequiescent condition (calm seas). In FIG. 4, the height E of the seafloorriser support can be reduced to about half the height shown.

[0029] The fluid transfer systems of FIGS. 1-6 result in severaladvantages over prior systems, especially in shallow water. The seafloorriser support 50, 110 of the conduit structure is fixed to the seafloorso its upper end 54 is fixed in position with respect to the seafloor.This fixes the end of the double catenary curve of the flexible riseropposite the vessel, high above the seafloor, using a reliable and lowcost structure. The rigid pipes preferably extend a plurality of metersalong the height of the structure. The system avoids or reduces the needfor distributed buoyancy modules, and avoids the need for an underwaterbuoy and for flexible risers that extend up from the seafloor to such abuoy.

[0030] Although particular embodiments of the invention have beendescribed and illustrated herein, it is recognized that modificationsand variations may readily occur to those skilled in the art, andconsequently, it is intended that the claims be interpreted to coversuch modifications and equivalents.

What is claimed is:
 1. An offshore fluid transfer system which includesa fluid-passing seafloor structure such as one connected to a seafloorwell or pipeline, a compliantly anchored floating structure such as avessel, at least one mooring line that is anchored to the seafloor andthat holds said floating structure in the vicinity of said seafloorstructure and at an initial position in a calm environment, and afluid-carrying conduit structure that extends up from said seafloorstructure to said floating structure, wherein: said conduit structureincludes a rigid seafloor riser support that extends a plurality ofmeters above the seafloor and that has lower and upper portions lyingrespectively at the seafloor and a plurality of meters above theseafloor, and said conduit structure includes a conduit that extendslargely upward along said seafloor riser support to said upper portionof said seafloor riser support, and that includes a flexible portionthat extends in a double catenary curve from said upper portion of saidseafloor riser support to said floating structure in said calmenvironment.
 2. The system described in claim 1 wherein: said sea has apredetermined sea depth above the seafloor at the bottom of saidseafloor riser support, and said seafloor riser support comprises arigid structure that extends above the seafloor by at least 10% of saidpredetermined sea depth.
 3. The system described in claim 2 wherein:said seafloor riser support extends above the seafloor by at least 20%of said predetermined sea depth.
 4. The system described in claim 1wherein: said conduit includes a rigid pipe of a length of a pluralityof meters that extends along at least part of the height of said risersupport, and said conduit flexible portion extends from said rigid pipeand from said seafloor riser support to said floating structure.
 5. Thesystem described in claim 4 wherein: said conduit structure includes acurved rigid pipe section with an inner end connected to an upper end ofsaid rigid pipe and an outer end connected to said flexible conduitportion, said pipe section inner end extending at an upward incline awayfrom rigid pipe and said pipe section outer end extending at a downwardincline away from said pipe section inner end.
 6. The system describedin claim 1 wherein: said seafloor riser support upper portion forms aconvexly rounded hose-supporting top surface, and said conduit flexibleportion includes a part that extends around said top surface and thatcan lift off said top surface.
 7. The system described in claim 1including: at least one pile that is driven into the seafloor and thatfixes said rigid seafloor riser support to the seafloor.
 8. An offshorefluid transfer system that lies in a sea of predetermined depth fortransferring fluid between a fluid-carrying seafloor structure that liesat the seafloor and a floating structure that floats at the sea surfaceand that is compliantly anchored to remain in the vicinity of theseafloor structure, the system including a conduit comprising a flexibleconduit member that extends along much of the sea depth, said conduithaving upper and lower ends coupled respectively to said floatingstructure and to said seafloor structure, comprising: a rigid framehaving a lower end fixed to the seafloor and an upper portion extendingabove the seafloor by at least 10% of said sea depth; and wherein saidconduit has a lower portion mounted on said frame, said conduit extendsalong said frame to said frame upper portion, and said conduit has aportion that includes said flexible conduit member that extends fromsaid frame upper portion to said floating structure.
 9. The systemdescribed in claim 8 wherein: said conduit includes a rigid pipe mountedon said frame and extending a plurality of meters along said frame, saidrigid pipe having an upper end connected to said flexible conduitmember.
 10. The system described in claim 8 wherein: said flexibleconduit member extends in a double catenary curve from said upperportion of said rigid frame to said floating structure, with a lowerportion of said flexible conduit member that extends from said upperportion of said rigid frame, extending at a downward incline away fromsaid upper portion of said rigid frame.
 11. The system described inclaim 8 wherein: said rigid frame has a convex upper surface with aradius of curvature of a plurality of meters, and said flexible conduitmember has a lower portion that lies on said convex upper surface. 12.The system described in claim 8 wherein: said rigid frame extends at anincline of more than 100 to the vertical and to the horizontal as viewedalong a lateral direction; and including a plurality of rigid pipemembers spaced in said lateral direction along said frame, one of saidrigid pipe member comprising at least a portion of said conduit lowerportion; a plurality of flexible conduit elements, including saidflexible conduit member, having lower ends extending from said pipemembers and having upper ends coupled to said floating structure. 13.The system described in claim 8 wherein: said rigid frame extends abovethe seafloor by at least 20% of said sea depth.
 14. An offshore fluidtransfer system that lies in a sea of predetermined depth fortransferring fluid between a fluid-carrying seafloor structure that liesat the seafloor and a floating structure that floats at the sea surfaceand that is compliantly anchored to remain in the vicinity of theseafloor structure, the system including a conduit comprising a flexibleconduit member that extends along much of the sea depth, said conduithaving upper and lower ends coupled respectively to said floatingstructure and to said seafloor structure, comprising: a rigid framehaving a lower end fixed to the seafloor and an upper portion extendingabove the seafloor; and wherein said frame has an upper surface and saidflexible conduit member has a lower portion that rests on said uppersurface and that is free to lift off and fall back onto said uppersurface.
 15. The system described in claim 14 wherein: said uppersurface has a portion of a length of over one meter that faces primarilyupwardly.
 16. The system described in claim 14 wherein: said uppersurface is convex with a radius of curvature of more than one meter.